A typical cellular mobile radio telephone system is controlled by at least one mobile switching center (also known as a mobile telephone switching office), at least one base station, and at least one mobile station. The mobile switching center constitutes an interface between the radio system and the public switching telephone network. The base station transmits information between the mobile stations and the mobile switching centers. Calls to and from mobile subscribers are switched by the mobile switching center. The mobile switching center also provides all signalling functions needed to establish the calls. In order to obtain radio coverage of a geographical area, a number of base stations are normally required. This number may range from, in the exceptional case, one base station, up to one hundred or more base stations in normal systems. The area is divided into cells, where each cell may either be serviced by a base station or may share a base station with a number of other cells. Each cell has an associated control channel over which control (nonvoice) information is communicated between the mobile units in that cell and the base station. Generally speaking, the control channel includes a dedicated channel at a known frequency over which certain information is communicated from the base station to mobile stations, a paging channel for unidirectional transmissions of information from the base station to the mobile stations, and an access channel for bidirectional communications between the mobile stations and the base station. These various channels may share the same frequency, or they may operate at different respective frequencies.
Three types of transmissions normally take place on the control channels between the mobile stations and the base stations. First, when a mobile station is originating a call, it sends an access request to the base station whose control channel has the strongest signal. This access request serves to inform the base station that the requesting mobile station needs to be assigned a voice channel over which the call can be connected. Second, when a mobile station is paged by a base station over the paging channel, indicating that the base station has a call to be completed to the mobile subscriber, the paged mobile station sends back a paging response on the access channel. Finally, when a mobile station travels from one cell to another, or for other reasons, the mobile station may send a registration access over the access channel to identify itself and its presence to the telephone exchange associated with the cell.
A mobile station in an idle state, i.e. with no active call taking place, will listen for pages and other information on the paging channel having the strongest signal strength. The mobile station will stay tuned to the paging channel until it either initiates an access, e.g. a registration, call access or paging response, or performs a rescan of all the paging channels within its range to determine if a new paging channel has a stronger signal strength than the current paging channel.
In conventional systems, there are three main reasons why an idle mobile station rescans the paging channel: loss of data, mobile station autonomous rescan, or a system message. Each of these is discussed below.
Regarding loss of data, the Total Access Communication System (TACS) standard, used mostly in the United Kingdom and China, indicates that if data cannot be correctly decoded for 5 seconds, the mobile station must rescan the paging channels. The Advanced Mobile Phone Service, Inc. (AMPS) specification, published by AT&T in 1983, indicates that a rescan must be done if five consecutive word synchronization sequences are received in error.
Regarding autonomous mobile station rescan, the AMPS standard specifies that each mobile station should have a rescan timer of five minutes which is reset every time the mobile station performs a rescan. When this timer times out, the mobile station carries out a rescan of the paging channels.
The third reason for a mobile station to rescan is if the system sends a rescan message in the Global Action Overhead Message--GAOM. When this message is received, all mobile stations receiving the message will rescan and select the paging channel with the strongest signal strength. This message could be sent, for example, from a base station immediately before it is shut down, due to, for example, hardware problems, or temporary maintenance. This would cause the mobile stations locked to the paging channel associated with the closing base station to lock onto the paging channels of surrounding cells instead.
In cellular radio systems, since the mobile station may move, the quality of the received signal may deteriorate if the mobile station leaves the base station's coverage area. As the quality of the received signal is reduced, the ability to correctly detect and interpret data messages is also reduced. This decreases the probability of, for example, receiving incoming calls.
By performing a rescan of all paging channels (which might be 21 channels, for example) and selecting the paging channel with the strongest signal strength, the word error rate of signals received at the mobile station may decrease, thereby increasing the probability of correctly receiving messages on the paging channel.
Because a rescan of the available paging channels normally takes a few seconds, an idle mobile station spends approximately at least one to two percent of its idle time doing rescans. During this time, the mobile station cannot respond to pages. In other words, at a given moment, a couple of percent of the idle mobile station population is not listening to the information sent from the system on the paging channel due to active rescans.
Another drawback associated with the need to use control channels in a cellular telephone system occurs when a mobile station locks onto one paging channel while traveling long distances (known as a sort of channel dragging). When the mobile station makes a system access the mobile station scans the access channels in order to select the strongest (which hopefully is the closest and/or the best to use) access channel. If the mobile station has rescanned the control channels recently, it is likely that the mobile station will choose the same control channel as the access channel with the strongest signal strength. If, on the other hand, the mobile station has moved a distance since the last rescan, chances are that the mobile station will choose another control channel as the access channel. The access channel may, in the worst case, belong to another system, another operator, and even another country. In most cases where problems develop, the access channel belongs to some system where the mobile station is not recognized as an authorized user of the system.
The unexpected access can lead to failure to complete the call as dialed or failure to connect the mobile station to the calling subscriber. If the mobile station would have discovered it had moved into another system and registered there, the newly entered system could then have more easily handled calls to and from this mobile station.